1. Field of the Invention
The present invention relates to a torque measuring device, and more particularly, to a torque measuring device based upon the principle of magnetostriction, or more precisely, an inverse magnetostriction, that is to surmise a stress condition of a ferromagnetic member from a distortion of the magnetic fluxes maintained by the ferromagnetic member according to an inverse application of the principle of magnetostriction.
2. Description of the Prior Art
A measurement of torque is required in various fields and is getting even more essential particularly in the field of controls of automobiles industrial robots, rolling mills of iron plants, production machines, OA machines and instruments, etc.
It is said that the magnetostriction was first found by J. P. Joule in 1842, and in 1960 O. Dahle proposed to measure a torque acting in a rotating shaft by an inverse application of the magnetostriction.
In measuring a torque applied to a ferromagnetic shaft member around its central axis based upon the inverse magnetostriction, since a distortion generated in the magnetic field of the shaft member by a torque applied thereto according to the inverse magnetostriction is generally very weak, it is required that a magnetostriction augmenting layer is formed along the outer circumferential surface of the shaft member by plating, plasma spraying, spattering, combustion coating, laser quick heating/chilling amorphous formation, etc. It was also proposed by Matsushita Electric Industrial Company to wind an amorphous magnetostriction ribbon around a shaft member in a lattice form with inclinations of .+-.45.degree. relative to its central axis. (Tech. Digest of 8th Sensor Symp. 1989 pp. 279-282)
However, when the shaft member for measuring a torque is for example, a steering shaft of an automobile, the shaft member is subjected to severe operating conditions such as changes of temperature over a wide range, and incessant vibrations and shocks, while, on the other hand, the requirement for the accuracy and reliability of the measurement is relatively high, with a simultaneous strong requirement for low costs of manufacture. The situation will be the same with respect to other members of automobiles of which it is desired to measure a torque applied therethrough, thereby rendering none of those surface treatments for the magnetostriction augmentation being employable for torque measurements of mechanical members of automobiles.
Although it is being researched to measure the torque by a normal steel shaft member with no such magnetostriction augmenting surface treatment, no success is yet heard because of a poor S/N ratio, i.e. ratio of effective signal to noise, available from an outer circumferential surface portion of a normal steel shaft member.
On the other hand, it has been proposed by Japanese Patent Laid-open Publication 5-196517 based upon U.S. Ser. No. 07/736,299 to mount a sleeve element around a shaft member by shrinkage, the sleeve element being made of a ferromagnetic material and magnetized to bear magnetic fluxes strong enough to generate a distortion thereof at an acceptable S/N ratio according to a torque applied to the shaft member. However, such can shrinkage assembled construction needs a special art of manufacture bound with a relatively high cost of manufacture, and yet not applicable to all kinds of shaft members, and further it is liable to a wide range of fluctuations in the performance according to respective products due to a small fluctuation in the diametrical size of the members to shrink and to be shrunk, in addition to changes of temperature which cause different thermal expansions of the shrinking and shrunk members.